All the carbohydrate foods we eat cause a release of glucose into the bloodstream—and a corresponding rise in insulin—but some raise glucose more than others. The glycemic index was created to differentiate those foods from each other: it’s a system of ranking foods containing equal amounts of carbohydrate according to how much they raise blood-glucose levels. It can help you, at least theoretically, to choose foods that have a gentler effect on blood-glucose levels.
How? The glycemic index (GI) categorizes foods based on how much a food containing 50 grams of carbohydrate raises blood-glucose levels after eating. A food with a GI of under 55 is considered low, while anything more than 70 is high. An even more precise measure is a food’s glycemic load (GL), which considers both a food’s GI and how much carbohydrate the food contains in a standard portion.
In general, most vegetables, whole grains, beans and other high-fiber foods are lower on the glycemic scale, while refined starches rank higher. But blood-glucose responses to foods can vary widely from person to person, particularly in people with diabetes. The scale can also have some strange inconsistencies: a Snickers bar (55) qualifies as a low-GI food, while fiber-rich black-bean soup (64) hovers close to the high-GI range. What’s more, most of us don’t eat foods in isolation, we eat combinations of different foods—and the other parts of a meal also influence the glycemic value. A high-GI potato becomes a low-GI meal if you add a pat of butter, for example.
Does paying attention to the glycemic scale help treat diabetes? The results from clinical studies haven’t been consistent. Some have shown modest benefits, while others have shown no effect.
Bottom line: The GI system is a helpful guide that can give you some general direction for making better choices among different carbohydrate foods, but don’t worry too much about the numbers. In truth, you don’t need a scoring system to tell you that whole grains, vegetables, beans and other high-fiber foods are great choices—and that processed and refined foods and sweets should be on the back burner.
Foster-Powell K, Holt S, Brand-Miller J. International table of glycemic index (GI) and glycemic load (GL) values: 2002. American Journal of Clinical Nutrition 2002; 76(1): 5-56.